1. Field of the Invention
The present invention relates to an apparatus and method for the electrical treatment of biological tissue. In particular, the present invention relates to an apparatus and method for providing a series of pulses for electrical stimulation of bone growth.
2. Discussion of Background
Human bone is a combination of organic and mineral components. The chief mineral constituent of bone is hydroxyapatite, a complex calcium phosphate (Ca.sub.5 (PO.sub.4).sub.3 OH) in crystalline form. Hydroxyapatite is piezoelectric: that is, it generates an electric charge or current when mechanically stressed. These electric signals are detected by nearby bone cells, stimulating them to deposit increased amounts of hydroxyapatite in response to the stress. This appears to be part of a biological feedback mechanism causing bone to be strengthened automatically at points of stress concentration. This mechanism also controls fracture healing. A similar feedback mechanism appears to control the mineral content of intact bone. When this mechanism breaks down, osteoporosis can result. In the case of a bone fracture, the normal healing process stops, resulting in a nonunion. Conventional treatment of nonunions usually involves surgical procedures, for example, "freshening" the broken ends of the bone or inserting pins to align the fracture. Surgery is, in effect, a new injury which restarts the dormant healing process. In many cases, however, conventional treatment is unsuccessful.
The weak electrical signals generated by bone were extensively studied and analyzed during the 1960's. During the 1970's, the results of these basic studies were applied in stimulating the healing of intractable nonunions and congenital pseudarthroses, with success rates of about 75%-80%. These results are not affected by factors such as infection, number of prior operative procedures, or soft-tissue or nerve defects. Other beneficial effects have been observed in the healing of soft-tissue injuries, including the healing of chronic skin wounds and the recovery of feeling in chronically numb skin grafts.
A waveform used for stimulation of bone growth is shown in FIG. 1. A series of pulses 10 consists of pulses 12, with pulse width 14 (5 msec), amplitude 16, and pulse interval 18 (61 msec) for a frequency of about 15 Hz. Each pulse 12 contains subpulses 20 with subpulse width 22 (200 .mu.sec) and subpulse interval 24 (28 .mu.sec) for a frequency of about 440 Hz. A waveform used for treatment of osteoporosis, shown in FIG. 2, consists of a series of pulses 30, with pulses 32 of pulse width 34 (380 .mu.sec), amplitude 36, and pulse interval 38 (13.5 msec) for a frequency of about 72 Hz. AC signals such as these, at levels comparable to normal piezoelectric signals (about 1 millivolt per centimeter) can increase the normal rate of bone healing and, more importantly, can stimulate healing in nonunions. AC signals several orders of magnitude more powerful have virtually the same effect as the weaker signals, indicating that a threshold effect is involved. DC signals, on the other hand, can damage soft tissues, and, if too strong, can lead to bone necrosis instead of healing.
Invasive techniques necessitate the implantation of electrodes below the skin (typically at the site of a nonunion), requiring surgery. See Adams (U.S. Pat. No. 4,602,638), and Hirshorn et al. (U.S. Pat. No. 4,414,979). Besides carrying some danger of infection, such procedures cause additional patient stress and require continuing professional care.
Much of the recent work in electrical bone growth stimulation has focused on noninvasive techniques using induced electric fields. The field generator and the patient are coupled by means of induction coils. See Welch (U.S. Pat. No. 4,672,951), and Niemi (U.S. Pat. No. 4,548,208). The coils are placed around the area to be treated, such as against the patient's skin or a plaster cast. Radio-frequency (RF) signals applied to the coils induce signals of similar form in bone and other tissues. This method is noninvasive, thereby simplifying patient care. Only AC signals are transmitted while DC is wholly blocked. However, such coupling is very inefficient at the low frequencies used. Elaborate circuitry is needed to drive the coils with a deliberately distorted waveform, or a waveform modulated with higher frequencies, so that the currents induced within the bone will approximate natural piezoelectric signals. Alternatively, simplicity is retained at the cost of added power drain by using different types of signals such as sine waves.
Coils, high-frequency generators, and modulating and driving circuitry all add to the weight, bulk, cost and power requirements of the signal generator. Generators presently in use may cost several thousand dollars each. Nominally portable equipment is driven by heavy, rechargeable battery packs with limited capacity; stationary devices may require connection to AC power. The inconvenience of using existing stimulators mandates intermittent use by patients, typically for only three to eight hours per day. Some models include circuitry to monitor use and help ensure patient compliance, which adds to the cost of the equipment. Often, coils must be custom-made for the individual patient. The signal generators must then be adjusted for optimum field distribution and adequate signal penetration into the bone. This also adds to the cost and may delay the beginning of treatment.
An ideal signal generator for the electrical stimulation of bone growth would be lightweight, compact, fully self-contained, inexpensive to build and maintain, safe for unsupervised home use, and require no external coils or battery pack. Such a stimulator could be taped directly to an arm or leg cast without adding significant weight or bulk. The stimulator would produce pulsed electric signals such as those shown in FIGS. 1 and 2 for treatment of fractures or osteoporosis, respectively or such other tissue treatment as may be desired. Signals would be delivered efficiently and uniformly throughout the treatment area, in a manner producing little or no distortion. No special traning would be required for use. Treatment would be continuous, minimizing problems of patient compliance. The stimulator would operate at low power levels, so there would be no shock hazard even in case of malfunction. Power would preferably be furnished by readily-available and inexpensive radio batteries.
No bone-growth stimulator presently available, or known to have been described in the medical literature, offers this combination of advantages.